1. Field of the Invention
The invention relates to a device for maintaining constant a certain viscosity of an adhesive, that is used for pasting the spine of an inner book or a book cover, by adding a diluent. The device comprises a rotating conveyor roll immersed in an adhesive container containing an adhesive and an applicator roll arranged downstream thereof which applies the adhesive onto the spine or the book cover of inner books passing by on a transport line. The device further comprises means for stirring and measuring the viscosity of the adhesive in the adhesive container as well as a control unit for comparing a measured value with a set-point value of the viscosity. The control unit with computing function is connected to the means for measuring the viscosity for the purpose of metering the diluent to be added to the adhesive.
2. Description of the Related Art
A device of this kind is commercially available from Kolbus GmbH+Co. KG in connection with a book finishing production line. Since a substantially homogenous state of the adhesive should be present for the measurement of the viscosity of the adhesive, a viscometer as well as a stirring apparatus are required. However, especially the use of a viscometer as a measured value transducer with its own control device results in relatively high costs for the aforementioned device, the price tag being at the moment approximately 15,000 German marks.
It is an object of the present invention to provide a device of the aforementioned kind which results in a reduced cost expenditure in comparison to known devices while providing reliable functionality and simplicity.
In accordance with the present invention, this is achieved in that the stirrer is embodied as a stirring apparatus configured to measure the viscosity of the adhesive contained in the container.
Preferably, the metering of the diluent can be carried out based on the changes of the electrical current consumption of the motor of the stirring apparatus with constant rpm (revolutions per minute).
As an alternative, the metering of the diluent can be carried out based on the change of the rpm of the stirrer with constant electrical current supply to the motor of the stirring apparatus.
When metering the diluent based on the change of the electrical current consumption, it is advantageous to provide a current measuring device in the current supply line of the motor and to connect the current measuring device to the control unit.
In the alternative metering of the diluent based on the change of the rpm of the stirrer with constant electrical current supply of the motor, it is expedient to provide a revolution counter (tachometer) coupled to the stirrer shaft.
The important features in this connection are the double function of the stirring apparatus and the use of an already present stored program control SPC of the device, and these features result in a considerable reduction of the cost of acquisition of such a device.